The present invention relates to the field of control of vehicle transmissions which are switchable between two wheel drive operation and four wheel drive operation, and in particular to the field of such control systems which operate automatically to switch the transmissions between a plurality of speed stages according to vehicle operational conditions.
Automatic transmissions for vehicles are per se known: such an automatic transmission provides one or the other of various speed stages (i.e. gearing ratios) between the vehicle engine and the wheels thereof, according to various operational conditions of the vehicle such as engine load and vehicle road speed, as well as according to the set position of a manual range setting means such as a lever which is set by the driver to one of several operational ranges such as "D" range, "2" range, and "L" range. The switching pattern between these speed stages is typically described by a shift pattern diagram.
Now, recently vehicles which can be optionally switched between a two wheel drive mode of operation and a four wheel drive mode have become more and more popular. Such a vehicle typically incorporates in its power train, typically after the main transmission unit thereof, a transfer transmission unit which can be switched either to provide power to the rear wheels only of the vehicle or to provide power to all the four wheels of the vehicle.
However, because the vehicle running characteristics are very much different between the four wheel drive operational mode of the vehicle and the two wheel drive operation mode, various problems have arisen.
Specifically, it has been known to incorporate into such a transmission unit also a means for providing an optional very low gear capability, i.e. a means for optionally selecting a speed reduction ratio which is substantially greater than unity such as, for example, 3.0. This is for providing a very strong capability for the vehicle to climb a hill or to escape from a muddy slough or the like. Therefore, if such an auxiliary transmission unit having a large speed reduction ratio is abruptly put into the lower speed stage operation when the vehicle is running at a relatively high speed, a large inertia shock will occur, thereby causing not only a great physical danger to the driver and passengers but also causing mechanical problems in the vehicle such as engine overrunning, wheel locking, etc. For switching such an auxiliary transmission unit between its high and its low speed, typically hydraulically driven wet multiplate clutch or brake, or alternatively a dog clutch operated by the vacuum pressure generated for the vehicle braking system has been used. Such a structure is disclosed in, for example, Japanese Patent Laying-Open Publications Nos. 55-152623 and 56-86825.
In order to avoid such problems due to the high inertia shock caused by abrupt downshifting of a high speed reduction ratio auxiliary transmission unit, it has been practiced to prevent the lower speed stage of the auxiliary transmission unit from being engaged, if the vehicle road speed has been over a certain predetermined value.
However, the above-mentioned problems due to the abrupt engagement to the lower speed stage of the auxiliary transmission depend not only upon the vehicle road speed at which the lower speed stage of the auxiliary transmission unit is engaged, but also upon other operational conditions of the vehicle such as engine throttle opening or accelerator pedal depression at that instant, the speed stage of the main transmission unit being engaged at the time, etc. Therefore, it is disadvantageous to cut off the utility of the auxiliary transmission unit by restricting the operational range thereof only in a region where vehicle road speed is less than a predetermined generally relatively low value.